This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Ancestral coatomer element 1 (ACE1) proteins assemble latticework coats for COPII vesicles and the nuclear pore complex. The ACE1 pro- tein Sec31 and Sec13 make a 2:2 tetramer that forms the edge element of the COPII outer coat. In this study, we report that the COPII accessory protein Sec16 also contains an ACE1. The 165-kD crystal structure of the central domain of Sec16 in complex with Sec13 was solved at 2.7-[unreadable] resolution. Sec16 and Sec13 also make a 2:2 tetramer, another edge element for the COPII system.Domain swapping at the ACE1[unreadable]ACE1 interface is ob- served both in the prior structure of Sec13[unreadable]Sec31 and in Sec13[unreadable]Sec16. A Sec31 mutant in which domain swap- ping is prevented adopts an unprecedented laminated structure, solved at 2.8-[unreadable] resolution. Our in vivo data suggest that the ACE1 element of Sec31 can functionally replace the ACE1 element of Sec16. Our data support Sec16 as a scaffold for the COPII system and a template for the Sec13[unreadable]Sec31 coat.